Sick normally means 'not feeling well' but in slang terms it is a synonym for 'awesome'
eg. Hope you have a sick weekend!
Note: Be wise in interpreting when people use that word. Someone with a pale face and runny nose saying "I'm sick" wouldn't be feeling awesome.

Sweet as bro is another synonym for 'awesome'
eg.
Jon: I bought you some lasagneGarfield: Sweet as bro!
Note: This slang is widely used in New Zealand as well. Grammatically speaking, there is supposed to be a comma after 'sweet as' because you are referring to the person you are talking to as 'bro'.

jigging means skipping a class or school
eg.
Julie: He was jigging his Biology class. I saw him at the shopping mall yesterday.

rad is another word meaning 'awesome'. It seems that Australians are bored of the word 'awesome' to come up with so many alternatives.

spider refers to float, the drink.
eg.
Diane: I drank a spider yesterday. Its so good, you should try it.
Insect: Uhh no thanks.

far out expresses surprise at some news, usually negative. Its similar to 'oh my gosh'
eg:
Joanna: Far out! I still can't figure out the answers even after doing it for hours!

mackers refers to McDonald's the fast food outlet
eg:
Ronald: Let's go to mackers to have a big Mac!

arvo is a shorter word for 'afternoon'. I heard someone say they are lazy to pronounce every syllable so they shorten it.
eg:
Janice: Hey the girls are meeting up tomorrow arvo..Wanna come?

ta is short for 'thank you'. How's that for lazy pronunciation?
eg:
Cashier lady at Wentworth: That's two dollars and forty cents change. Ta!